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Embryology and Anatomy of Teeth and Periodontal Tissues



2 . Embryology and Anatomy of Teeth and Periodontal Tissues

In order to understand wound healing associated with transplanted teeth, it isimportant to have a working knowledge of the embryology and anatomy

of teeth and periodontal tissues. The following represents current knowledge in theseessential areas.

Embryology of Teeth and Periodontal Tissues1-3The beginning of the tooth germ occurs when neural crest cells migrate to the locationof the future dental arch producing a thickening of the epithelium and forming thedental mesenchyme (Fig 2-1). Under the influence of the dental mesenchyme, anectodermal bud and, subsequently, enamel develop (Fig 22). The epithelial cellsdifferentiate into inner and outer enamel epithelium. Neural crest cells become dental papilla at the convex area of the inner enamel epithelium then differentiate intoodontoblasts and undifferentiated mesenchyme cells and fibroblasts (Fig 2-3). The

enamel and dental papilla are held in fiber-rich mesenchymal cells (Fig 2-3). Thissaclike structure is called the dental sac and differentiates into the periodontalligament.

The development of hard dental tissues starts with the development of dentin. The preameloblasts, arising from the cells of the inner enamel epithelium, induce thedifferentiation of odontoblasts, which originate in the peripherally located cells of thedental papilla. Differentiation of the preameloblasts into ameloblasts follows and thesecells begin the synthesis of the enamel matrix. The development of the roots beginsafter enamel and dentin formation has reached the future cemento-enamel junction(CEl). The inner and outer enamel epithelia join to form Hertwig's epithelial rootsheath, which molds the shape of the roots and initiates dentin formation. As the firstlayer of dentin is laid down, the epithelial root sheath loses its continuity and migrates

apically (Figs 2-4 and 2-5). Its remnants persist as epithelial rests of Malassez in the periodontal ligament (Fig 2-6).

Periodontal tissue is derived from the dental sac proper. It has recently beensuggested that enamel matrix protein (mainly amelogenin) secreted from inner cells of Hertwig's epithelial root sheath plays an important role.4 Cells derived from the dentalsac proper differentiate into cementoblasts or fibroblasts induced by enamel matrix protein, and cementum is deposited on the root surface and Sharpey's fibers embedinto this new cementum. In the meantime, osteoblasts, which were differentiated fromthe dental sac proper, form the alveolar bone proper on the inner surface of thealveolar process. Sharpey's fibers are embedded in the alveolar bone proper as well(Fig 2-6). Thus, Sharpey's fibers serve as attachment for the tooth to the surrounding

bone (Fig 2- 7) .

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Embryology of Teeth and Periodontal Tissues

Figs 2-1 to 2-3 Developm.ent of tooth germ (courtesy of Dr Takeyasu Maeda, NHgata

lJniY_r_ity School of Dentistry, Department of Oral Anatomy).

Fig 2-1 (above left) Neural crest cell migration and

formation of dental ledge. Thickening of the epithelium

occurs in the region of the future dental arch after

migration and proliferation of mesenchymal cells derived

from the neural crest (NC); then the dental organ (DO) is

formed.

Fig 2-2 (above right) Tooth germ at 9 to 10 weeks of

embryonic life. The dental organ proliferates and

differentiates, forming enamel.

Fig 2-3 (right) Twelfth week at bell stage. Inner enamel

epithelium (a), outer enamel epithelium (b), stellate

reticulum (c), and dental papilla cells (d) can bedistinguished, and dental germ is held in the dental sac

(e). The dental sac consists of fiber-rich mesenchymal

tissue.

Figs 2-4 and 2-5 Development of roots (courtesy of Dr Takeyasu Maeda, Niigata University

School of Dentistry).

Fig 2-4 (near right) Formation of Hertwig's epithelial root

sheath and root dentin. After completion of formation of

enamel and dentin of the crown, the inner and outer

enamel epithelia grow apically, making contact with each

other. These two extended layers of epithelial cells

become Hertwig's epithelial root sheath, which helps to

form the outer shape of roots and induce the dif-

ferentiation of odontoblasts.

Fig 2-5 (far right) Epithelial dissepiment. The area of

Hertwig's epithelial root sheath folded into dental papilla is

called the epithelial diaphragm.

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Inside Hertwig's epithelial root sheath, odontoblasts differentiated from dental pulp cells form root dentin. Cementum is deposited on

the surface of newly formed dentin by cementoblasts differentiated from cells of the dental sac. Sharpey's fibers are embedded in cemen tum by

fibroblasts derived from the dental sac. At the same time, the alveolar bone proper is formed by osteoblasts from the dental sac;Sharpey's fibers are embedded into this new bone.

.................................------------_------------------,__- -- -- -- - ...,....._ _ - -

_ periodontal tissues of developing teeth.I

.------------------------'"...., ... . _ - -_ """ -- -- - - - - - -

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"



Anatomy of Teeth and Periodontal Tissues

The periodontal ligament contains many blood vessels M. Asingle layer of cementoblasts (C) is observed on the surface

of the cementum, and epithelial rests of Malassez (M) arevisible nearby. Osteoblasts (0) align on the surface of thealveolar bone proper (A), and periodontal ligament fibers(P) run between the bone and cementum.

Dentin Treriodontalligament Alveolar bone

Cementum

Anatomy of Teeth and Periodontal Tissues

The anatomy of fully erupted teeth and periodontal tissuesl-3,S is illustrated in Fig8. The CEJ is located about 1 mm coronal to the crest of the alveolar bone. Junctionepithelium exists 1 mm coronal to the CEJ, which is attached firmly to the too(epithelial attachment) by hemidesmosomes. The root apical to the CEJ is attachedgingival connective tissue or alveolar bone through cementum and Sharpey's fibeThis type of attachment is called connective tissue attachment. A band of soft tissattachment between the base of the gingival sulcus and the alveolar crest, whichcomposed of approximately 1 mm of attachment epithelium and 1 mm of connecttissue attachment (3 mm, iJ).cluding 1 mm of sulcus), is referred to as the biologwidth6-10 (Fig 2-8d). The biologic width must be taken into consideration because plays an important role in maintaining the health of the periodontal tissues. Alveo

bone is composed embryologically of two parts: the alveolar process and the alveo bone proper. The alveolar process is derived from basal bone and the alveolar bo proper is the compact bone that is deposited on the inner surface of the alveo process by osteoblasts derived from the periodontal ligament. The alveolar bo proper is the cortical bone forming the inner surface of the alveolar socket andobserved as lamina dura in radiographs (Fig 2-8b). Gingiva and alveolar mucocover the teeth and the alveolar bone. The alveolar mucosa is unkeratinizepithelium covering muscles and alveolar bone. Gingival tissue is keratinized aconsists of attached gingiva, which is attached to teeth or alveolar bone, and frgingiva, which covers the sulcus surrounding teeth (Figs 2-8a and 2-8c).

_@ - _

Fig 2-7 ligament (courtesy of Dr Takeyasu Maeda,of Dentistry).




F'ig 2-8 Al'"latdnJy of teeth and supporting tissues.

Figs 2-8a to 2-8d A fully erupted permanent tooth and its supporting tissues. Abone consists embryologically of the alveolar bone proper and the alveolar process. The

process is derived from the basal bone and the alveolar bone proper is derived fromodontal ligament. The alveolar bone proper is observed as lamina dura on the radiograph.consists of free gingiva and attached gingiva transforming to alveolar mucosa. The CEJ is mm coronal to the crest of the alveolar bone, and 1 mm of connective tissue attachment extenthe crest of the alveolar bone. Moreover, 1 mm of junctional epithelium and 1 mm of gingivare formed coronal to it. Because these connective tissue attachments and junctional epithelthe crest of bone are observed on almost all tooth surfaces, they determine biologic width. I

periodontal tissue, biologic width is 3 mm after adding 1 mm of gingival sulcus.

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